<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
        form {
            display: flex;
        }

        input{
            flex-grow: 1;
        }
    </style>
</head>
<body>
    <form >
        <input type="email" name="email" >
        <button type="submit">Send</button>
    </form>
</body>
</html>